1. Field of the Invention
The present invention relates to an apparatus for controlling a quantity of light to be projected on an electrostatic latent image bearing member such as photoreceptor which is generally used in an electrophotographic image forming apparatus such as copier, thermal printer, inkjet printer and facsimile machines.
2. Description of the Prior Art
An image forming apparatus such as an electrophotographic copy machine comprises an electrostatic latent image bearing member, i.e., photoreceptor having a photosensitive layer thereon. The photosensitive layer has a thickness that decreases with time of use, accompanied by decrease in sensitivity. In order to offset the decrease in sensitivity to thereby keep making an image having a suitable density, there is proposed a method in which the quantity of light emitted from an exposure light-source is controlled by adjusting a voltage to be applied to the light source in accordance with the reduction in thickness of the photosensitive layer, i.e., the decrease of sensitivity thereof. In reality, however, the method may not accomplish its purpose, because an available controlling range of the light quantity based on the voltage adjustment is restricted and an excessive reduction in voltage results in a unstable and uneven exposure.
Further, although a new photoreceptor having a thicker photosensitive layer formed thereon is proposed for the purpose of extending a lifetime thereof, the above mentioned restriction in voltage to be applied to the light-source renders the photoreceptor unable to work over its long service life.